1. Field of the Invention
The present invention relates to a pneumatic tire and, more specifically, to a run-flat tire that enables safe drive over a long distance even with a decreased inner pressure at the time of puncture.
2. Description of the Background Art
Generally, at the time of a tire puncture, a side-wall portion 31 having low rigidity of the tire is folded and protruded in the widthwise direction of the tire as shown in FIG. 6, and a tread portion 32 moves inward in the radial direction of the tire, so that the tire becomes flat. When running continues, a tire bead portion 33 falls into a rim well 34, possibly causing a hazardous state in which the tire drops off from the rim, disabling steering, or an inner upper end 33a of the bead portion is repeatedly subjected to severe friction with the tire inner region 32a of the tread portion, damaging the tire.
Conventionally, as a structure of a so-called run-flat tire enabling safe drive even at the time of tire puncture, the following structures have been proposed: a structure in which a crescent shape reinforcing rubber layer is arranged in contact with an inner side of a tire carcass over the bead portion to a shoulder portion, with its thickness gradually reduced toward opposing ends, or a structure in which a reinforcing rubber is arranged from the bead portion to an end of the thread portion between the carcass body and a folded portion thereof (Japanese Patent Laying-Open No. 10-244817); and a structure in which two-layered reinforcing rubber is arranged between a plurality of carcass plies or reinforcing plies (U.S. Pat. No. 5,427,166).
The above described prior art contemplates to reduce tire deformation at the time of puncture to avoid the hazardous state described above, by increasing rigidity of the side-wall portion by the reinforcing rubber. However, the reinforcing rubber layer arranged on an inner peripheral side of the carcass body portion as described above increases local strain at the side-wall portion at the time of tire puncture, and therefore damage to the side-wall portion of the tire cannot be alleviated. In order to further enhance the effect of reinforcement, the thickness of the reinforcing rubber layer must naturally be increased. This, however, increases the weight of the tire, and decreases fuel efficiency of the vehicle.
An object of the present invention is to provide a pneumatic tire, and particularly a run-flat tire, having tire weight effectively reduced and enabling safe running over a long distance even at a time of tire puncture.
The present invention provides a pneumatic tire, including at least one carcass ply having a steel cord arranged substantially in a radial direction of the tire, folded turned up around a bead core, a belt layer arranged on an outer side of a crown portion of the carcass ply to have an overlapping portion with the folded portion of the carcass, a first reinforcing rubber layer arranged between body portion of the carcass ply and the folded portion of the carcass ply, and a second reinforcing rubber layer arranged inside the body portion of the carcass ply, over a region corresponding to a tire shoulder portion to a bead portion, with its thickness gradually reduced from a central portion to opposing end portions, wherein overlapping width W2 from an upper end position RF of the second reinforcing rubber layer to an end BE of the belt layer is 7% to 33% of the width BW of the belt layer, and cross sectional widths G1W, G2W, and G3W representing minimum widths between an inner surface of the tire and respective side-wall outer surface positions G1, G2 and G3 at portions of xc2xcGL to xc2xeGL corresponding to cross sectional height GL from an upper end of a bead wire to a tire outer peripheral end divided equally by 4 satisfy the following relations:
G3W/G1W=0.85 to 0.95, and
G2W/G1W=1.0 to 1.05.
In the present invention, preferably, respective cross sectional widths G1W, G2W and G3W at respective side-wall outer surface positions G1, G2, and G3 corresponding to xc2xcGL to xc2xeGL, and widths G1a, G2a, and G3a of the second reinforcing rubber layer at respective positions satisfy the following relations:
G1a/G1W=0.30 to 0.55
xe2x80x83G2a/G2W=0.38 to 0.48, and
G3a/G3W=0.20 to 0.30.
Further, it is desired that the first reinforcing rubber layer consists of two layers including a hard rubber layer extending in a direction from an upper side of the bead core to the side-wall portion, having JISA hardness of 80 to 95, and a soft rubber layer extending from an upper side of the hard rubber layer to a vicinity of an end portion of the belt layer, having JISA hardness of 65 to 80.
Further, overlapping width W1 of the upper end RE of the first reinforcing rubber layer and the end BE of the belt layer is in the range of 3% to 20% of the width BW of the belt width.
According to another aspect, the present invention provides a pneumatic tire, including at least one carcass ply having a steel cord arranged substantially in a radial direction of the tire, folded turned up around a bead core, a belt layer arranged on an outer side of a crown portion of the carcass ply to have an overlapping portion with the folded portion of the carcass, a first reinforcing rubber layer arranged between body portion of the carcass ply and the folded portion of the carcass ply, and a second reinforcing rubber layer arranged inside the body portion of the carcass ply, over a region corresponding to a tire shoulder portion to a bead portion, with its thickness gradually reduced from a central portion to opposing end portions, wherein overlapping width W1 of an upper end position RE of the first reinforcing rubber layer and an end BE of the belt layer is in the range of 3% to 20% of the width BW of the belt layer, an overlapping width W2 of an upper end position RF of the second reinforcing rubber layer and the end BE of the belt layer is 7% to 33% of the width BW of the belt layer, and thickness RW of the second reinforcing rubber layer on a line TL connecting the end BE of the belt layer and a tread end TE is 5% to 25% of the total thickness SW of the tire. Preferably, the first reinforcing rubber layer consists of two layers including a hard rubber layer extending in a direction from an upper side of the bead core to the side-wall portion, having JISA hardness of 80 to 95, and a soft rubber layer extending from an upper side of the hard rubber layer to a vicinity of an end portion of the belt layer, having JISA hardness of 65 to 80. Further, preferably, JISA hardness of the second reinforcing rubber layer is in the range of 65 to 80.
Further, in the pneumatic tire of the present invention, the width Ga of the second reinforcing rubber layer, the width Gb of the soft rubber layer and the width Gc of the hard rubber layer of the first reinforcing rubber layer and the width Gd of the side-wall on the lines of minimum cross sectional widths G11W, G12W, G13W and G14W representing minimum widths from respective inner liner positions G11, G12, G13 and G14 to a tire outer contour line at positions ⅙GL to {fraction (4/6)}GL corresponding to the cross sectional height GL from a lower end of the bead wire to a tire outer peripheral end divided equally by 6 satisfy the following relations:
At G11W, G11axe2x89xa6G11d less than G11c,
At G12W, G12b less than G12axe2x89xa6G12c less than G2d,
At G13W, G13a≈G13b≈G13d, and
At G14W, G14a≈G14b less than G14d.
The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.